A document is typically edited on a computer and then distributed in the form of electronic data. In the case of a hardcopy document, a portion including secret information such as private information may be blackened out to prevent the secret information from being leaked.
An electronic data document may be typically distributed in a data format that does not permit the document to be edited, with a secret information portion thereof masked. The leakage of secret information is thus controlled. Also to prevent information leakage, an electronic data document may be distributed with the masked secret information portion password-locked in an uneditable state.
However, the distribution of a document in an uneditable data format presents a disadvantage that a user having received the document cannot reuse the document even if the portion of the information the user wishes to use is a non-secret portion of the document.
The distribution of a document in an uneditable data format in a password-locked state presents another disadvantage that, although the document can be reused by unlocking the password, only a limited number of users who know the password can edit the document.
Japanese Patent Laid-Open No. 3-209577 discloses a technique in which a secret information portion of an electronic data document is converted into information different from the original information in order to prevent the secret information from being leaked in the course of distribution of the document.